The manufacture of semiconductor devices typically involves applying a layer of a photosensitive substance (a photoresist) to the surface of a target wafer. The photoresist is exposed to light in a selected pattern using a photomask, and the photoresist is then developed to leave exposed regions of the wafer. Typically, the exposed regions are subsequently etched away or otherwise modified, and the residual photoresist is removed. The pattern of the photomask typically possesses extremely fine details, and the presence of even tiny particles on the surface of the photomask can interfere with the accurate reproduction of the pattern on the target wafer.
To minimize particulate contamination at the mask surface, optical pellicles have been developed that protect the photomask. An optical pellicle includes a frame-mounted transparent membrane, and is attached to the photomask surface, so that contaminating particles fall onto the pellicle membrane and not the surface of the photomask. The pellicle frame holds the pellicle membrane at a sufficient distance above the mask surface so that any particles that may fall upon the membrane lie outside the focal plane of the illuminating light, and so fail to interfere with the projected mask pattern. The use of optical pellicles in semiconductor manufacture has helped mitigate the effects of contamination by dust and other particulates, and has become widespread in the industry.
Multiple barriers to achieving fast, cost-effective, high-quality photolithographic reproduction remain. A first barrier is contamination of the photomask. In particular, when a pellicle is removed from a photomask (e.g., because the pellicle has reached the end of its operational lifetime), particulate contaminants are often generated. These contaminants may include small fragments of the adhesive traditionally used to secure the pellicle to the photomask, and particulate generated by mechanical contact between tools used to remove the pellicle and the pellicle and/or photomask, for example. Additionally, solvents typically included in pellicle adhesives may outgas when the pellicle is exposed to inspection or exposure illumination sources, which may distort the electromagnetic radiation as it passes through the pellicle and the photomask.
A second barrier is the inadequacy of traditional methods and pellicle materials in high energy photolithography. Demand for smaller, faster, and more powerful microprocessors has required the semiconductor industry to fabricate ever smaller and faster semiconductor circuits. Manufacturing techniques have advanced to the point that the size of the circuit being produced is effectively limited by the wavelength of light used in the photolithographic process, with shorter wavelength illumination permitting finer details in the resulting circuit structure. Photolithography using 248 nm, 193 nm, and 157 nm illumination (in the deep ultraviolet, or DUV, range), as well as photolithography using 13.6 nm illumination (in the extreme ultraviolet, or EUV, range), are known.
However, the organic materials typically used as pellicle membranes tend to break down under DUV and EUV illumination, and thus cannot protect the photomask during photolithography processes at DUV and EUV wavelengths. Attempts to develop a pellicle membrane that can withstand EUV illumination (for example, a metal mesh) remain hindered by material brittleness, the challenges of eliminating outgassing contaminants, and the lack of sufficiently reliable procedures for cleaning and manufacturing. Additionally, such pellicle membranes, although transparent to EUV illumination, are not transparent at the wavelengths used to inspect a photomask for defects (typically around 193 nm) prior to EUV photolithography. Thus, such pellicle membranes must be removed during inspection, generating particulate contamination and risking damage to the photomask.